The invention relates to a process for producing steel melts, in which, to desulfurize iron melts, a desulfurization slag of the following chemical analysis
SiO2 max. 20 % by weight
Al2O3 max. 50% by weight
SiO2+Al2O3+TiO2=5-40% by weight
FeO max. 2.0% by weight
MnO max. 1.5% by weight
CaO+MgO+BaO+Na2O+K2O=25-65% by weight
MgO max. 20% by weight
Na2O+K2O max. 10% by weight
CaF2=0-60% by weight
CaO+MgO+BaO+Na2O+K2O+CaF2=50-85% by weight                     CaO        +        MgO                              SiO          2                +                  0.5          ⁢                      xe2x80x83                    ⁢                      Al            2                    ⁢                      O            3                                ⁢          min      .              xe2x80x83            ⁢      2                                                Na            2                    ⁢          O                +                              K            2                    ⁢          O                            SiO        2              ⁢          max      .              xe2x80x83            ⁢      1      
and impurities from the raw materials is brought to a temperature of from 1400-1800xc2x0 C. in a desulfurization vessel through heating of the desulfurization slag, preferably by means of electrodes which are immersed in the desulfurization slag, and this desulfurization slag is used to desulfurize the sulfur-containing iron melt, which is then poured off, as far as possible without any slag, either discontinuously or continuously below the desulfurization slag, the ratio of iron melt to desulfurization slag not exceeding the value of 10:1 parts by weight, and the desulfurization slag being regenerated continuously and/or discontinuously for a further pig-iron desulfurization treatment, and then a steel melt being produced from the iron melt.
A process of this type is known from EP-0 627 012 B1. This known process has particular advantages over other known desulfurization processes, including, inter alia, the avoidance of the amounts of slag produced which previously had to be landfilled or reprocessed in a complex way, the avoidance of expensive desulfurizing agents, such as lime, carbide, magnesium, etc., the avoidance of iron losses which occur during the deslagging involved in pig-iron desulfurization, and the avoidance of a temperature drop in the pig iron during the desulfurization. The pig iron which has been desulfurized down to very low S contents using the process which is known from EP-0 627 012 B1 is used as a starting material for steelmaking and, for this purpose, is used, for example, in a converter or an electric furnace.
The object of the invention is to develop this process further in such a way that, during a ladle treatment of the crude steel produced from the desulfurized pig iron, no additional slag components are required, so that associated problems of introducing additives and disposing of the ladle slag are eliminated. It is to be possible to carry out the ladle treatment with a very small supply of energy, and steel losses such as those which usually occur during casting after the ladle treatment (residual steel in the steel-casting ladle), are to be minimized or avoided.
According to the invention, this object is achieved by the fact that for a subsequent ladle treatment of a crude steel melt, a partial amount of the desulfurization slag from the desulfurization vessel is introduced into a steel-casting ladle which is to receive the desulfurized iron melt, which has been converted into a crude steel melt, and this partial amount is recirculated after the ladle treatment and after the steel melt formed in this way has been poured off. The slag from the desulfurization vessel, which is used to desulfurize the pig iron, which is fed to the steel-casting ladle therefore entirely replaces the components of the synthetic slag which, according to the prior art, have to be supplied to a ladle furnace. Since this slag is fully recirculated, i.e. is reintroduced into the desulfurization vessel, there is no landfill material produced in connection with the ladle metallurgy; the desulfurization slag is passed through a closed circuit.
Entrained slag from the converter or electric furnace and deoxidation products are incorporated in the circuit, since they are combined with the desulfurization slag during or after tapping. This additional amount causes the amount of slag in the pig-iron desulfurization to rise gradually, and the excess quantity may advantageously be utilized together with the slag which is formed during the steel making.
Advantageously, after the steel melt has been poured off, a residual amount of the steel melt which remains in the steel-casting ladle is recirculated together with the desulfurization slag which is to be recirculated, and is introduced into the iron melt which has not yet been desulfurized in the desulfurization vessel, with the result that for quality reasons the amount of residual steel can be kept at a greater level compared to the prior art. Entrainment of slag during casting of the steel can be prevented more reliably or ruled out altogether. This is particularly advantageous if continuous casting is used, since in this way it is very easy to prevent slag from penetrating into a tundish of a continuous-casting installation.
It is known from DE-195 46 738 C2 to carry out a desulfurization treatment with the aid of a desulfurization agent which is in powder form, in which case, during a ladle metallurgy treatment carried out on a steel melt, the slag which is produced in the ladle furnace, after the steel melt has been cast, is introduced, together with a residual amount of steel, into a hot pig-iron charging ladle, and further pig-iron melt which is to be desulfurized is added to this ladle. Then, the pulverulent desulfurizing agent is added, the melt together with the desulfurizing agent being made turbulent with the aid of a carrier gas. However, in this known process it is necessary to use the usual ladle slag components for the ladle metallurgy, which represents a considerable financial outlay both for the production of the ladle slag and its disposal.
According to the invention, the partial amount of the desulfurization slag which is removed from the desulfurization vessel and introduced into the steel-casting ladle is expediently less than 30 kg/t of iron melt, preferably less than 20 kg/t of iron melt.
According to a preferred embodiment, a partial amount of the desulfurization slag is removed from the desulfurization vessel and introduced into the steel-casting ladle after the regeneration. This is particularly advantageous if very low sulfur contents are important, since the slag from the pig-iron treatment vessel has a high slag-uptake capacity.
Another preferred embodiment is characterized in that a partial amount of the desulfurization slag is removed from the desulfurization vessel and introduced into the steel-casting ladle before the regeneration. In this case too, it is possible to considerably reduce the sulfur contents in the steel, but the partial amount of the desulfurization slag can be removed from the desulfurization vessel immediately after the pig-iron desulfurization which takes place in this vessel. On account of the large volume of slag for the pig-iron treatment, the sulfur level is still relatively low, or the slag is able to take up sulfur from the steel, even before the regeneration of the slag.
It is advantageous if the partial amount of the desulfurization slag which is removed from the desulfurization vessel is transferred to the steel-casting ladle with thermal insulation and in the liquid state, in which case, advantageously, the desulfurization slag which has been removed from the steel-casting ladle and is to be recirculated into the desulfurization vessel is also conveyed, i.e. recirculated, to the desulfurization vessel with thermal insulation and in the liquid state.
The desulfurization slag is expediently transferred by means of transfer vessels which have been suitably preheated and insulated. To reduce the outlay on this transfer, it is expedient for slag (if appropriate together with residual steel) from a plurality of steel batches to be transferred back and forth together in a single transfer vessel, if appropriate in combination with a ladle-heating burner.
The partial amount of slag may be poured into the steel-casting ladle before or after the crude steel is added to the steel-casting ladle, adding the steel later having the advantage of bringing about thorough turbulence and therefore possibly an additional desulfurization reaction, specifically even when the slag already has a relatively high sulfur content. Moreover, this promotes the separation of nonmetallic inclusions which are produced by deoxidation and therefore improves the purity of the steel.